Create a low poly mesh with high detailed textures?
I'll show the most simple way to bake a normal map and then convert it to another format so that it's more widely compatible with game engines.  

I am leaving out ALL theory and best-use scenario information since this topic almost always seems to cause arguments for some silly reason.

I'll just show how to bake an Object Space map and then convert it into a Tangent map since this method of creating a Tangent space map isn't widely known and tends to be very useful especially for a situation like the one I'm showing here.  Instructions for other methods are easy to find.

Have your simple model ready and while in Edit mode press u and select-> 'Smart UV Project', make sure there is a bit of space between all the UV islands as shown in the next image.  Then press 'New' and add a texture which will be baked to.



Now if you add a bunch of Modifiers to increase the geometry and shape it a bit, such as Subdivision, Displace, (Bevel sort of works for this), you can then press 'Bake' and you will have a normal map.(there is a link to a simple modifier tutorial at the end)

Now you can add a new material then move one tab over to the textures and add one of those as well.  The relevant settings are shown below.



Now to convert this to a Tangent Space map,  First of all, deactivate all of the modifiers by unchecking the eyeball and camera icons, otherwise the next stage will fail.  We want the normals to come from the map that we already baked.



Now add another texture using the UV image editor and make sure that you are in Edit mode and have all the faces selected when you add the texture.  This way of adding a texture is primarily used for baking in Blender Internal, I mention this because many beginners think that they have added a texture to their model but that's not really what's happening here.  There is a big difference between adding a bake-to texture and adding a material texture.  

Anyways, with this second texture added, go over to the bake section and change the setting to bake to a Tangent map. 



Press bake and you should now have a normal map that looks mostly pale blue.  Instead of adding a second texture slot, you can just replace the Object Space map with the Tangent space map and change the Image Sampling section to Tangent map as I've circled in the image.





The modifier stack is based on the following webpage.

http://www.evermotion.org

Create Stone Wall in Blender - Tip of the Week

http://www.evermotion.org/tutorials/show/9267/create-stone-wall-in-blender-tip-of-the-week

Here's the .blend file.  The images are not packed so even though they should be listed, you'll have to recreate them.  http://www.pasteall.org/blend/34142